[clang] [ASTWriter] Do not allocate source location space for module maps used only for textual headers (PR #116374)

[Ilya Biryukov via cfe-commits]

ilya-biryukov wrote:

@jansvoboda11 I plan to land this tomorrow/slightly later as I believe all concerns are addressed. And I would be happy to revert if it causes problems or follow up with fixes if you'll have more comments.

However, feel free to say something until that happens if you feel like I'm missing something.

https://github.com/llvm/llvm-project/pull/116374